TAPE UNIT SCSI COMMANDS

M2488 PRODUCT GUIDE

4-3.1 Command Description Block Format

A command is executed by sending a Command Descriptor Block (CDB) to a target. See the exam- ple for a six-byte command below. The CDB may be 6, 10 or 12-bytes in length depending on the type of command. Some commands have additional fields which are described in the individual com- mand. Use the description in Table 4-2 to interpret the common fields of the commands that follow.

BITS

BYTES

7

6

 

5

 

4

 

3

 

2

 

1

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

Group Code

 

 

 

 

 

Command Code

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

 

 

LUN

 

 

 

 

 

 

 

Reserved

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

 

 

 

 

 

 

Reserved

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5

 

 

 

 

 

Reserved

 

 

 

 

 

 

Flag

 

 

Link

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: Changeable fields in the CDB are shaded.

 

Table 4-2. CDB Field Description

 

 

FIELD

DESCRIPTION

 

 

Operation Code (Op Code)

All commands have one Op Code in byte 0. This is the operation to be performed.

Consists of a command code and a group code.

 

 

 

Command Code

Identifies the command to be executed.

 

 

 

Specifies a CDB format, length in bytes, and classifies the type of operation.

Group Code

For CDBs using reserved group codes 3 and 4, the controller will accept only one byte

of CDB, then go to the Bus Free phase. The host system should not send more than one

 

 

byte of CDB in this case.

 

 

 

The LUN is defined in the IDENTIFY message. The target ignores the logical unit

 

number specified within the CDB. The LUN in the CDB should be zero.

LUN (Logical Unit Number)

The LUN field is included in the CDB for compatibility with some SCSI-1 devices.

 

This field may be reclaimed in SCSI-3. New implementations should use the outbound

 

IDENTIFY message, which is mandatory in SCSI-2, to establish the I_T_L nexus.

 

 

Reserved

Set aside fields for future use. Default value is 0 for all bits.

 

 

Control

Last byte of all commands. Used for linked commands. Consists of a link bit, a flag

bit, and reserved bits.

 

 

 

 

Used to continue I/O processes across multiple commands. Creates an automatic link to

Link Bit

the next command upon successful completion of the current command. If Link=0, then

no link to next command. If Link=1, then an automatic link to the next command

 

 

occurs.

 

 

 

Interrupts the initiator between linked commands. Used with the link bit to specify

Flag Bit

which message is returned to the initiator when the command completes without error;

if Flag=0, LINKED COMMAND COMPLETE message; if Flag=1, LINKED COM-

 

 

MAND COMPLETE (with Flag) message.

 

 

4-4

CG00000-011503 REV. A

April 1997

Page 86
Image 86
Fujitsu M2488 manual Command Description Block Format, CDB Field Description